This PR removes boxed closures from the language, the closure type syntax (`let f: |int| -> bool = /* ... */`) has been obsoleted. Move all your uses of closures to the new unboxed closure system (i.e. `Fn*` traits).
[breaking-change] patterns
- `lef f = || {}`
This binding used to type check to a boxed closure. Now that boxed closures are gone, you need to annotate the "kind" of the unboxed closure, i.e. you need pick one of these: `|&:| {}`, `|&mut:| {}` or `|:| {}`.
In the (near) future we'll have closure "kind" inference, so the compiler will infer which `Fn*` trait to use based on how the closure is used. Once this inference machinery is in place, we'll be able to remove the kind annotation from most closures.
- `type Alias<'a> = |int|:'a -> bool`
Use a trait object: `type Alias<'a> = Box<FnMut(int) -> bool + 'a>`. Use the `Fn*` trait that makes sense for your use case.
- `fn foo(&self, f: |uint| -> bool)`
In this case you can use either a trait object or an unboxed closure:
``` rust
fn foo(&self, f: F) where F: FnMut(uint) -> bool;
// or
fn foo(&self, f: Box<FnMut(uint) -> bool>);
```
- `struct Struct<'a> { f: |uint|:'a -> bool }`
Again, you can use either a trait object or an unboxed closure:
``` rust
struct Struct<F> where F: FnMut(uint) -> bool { f: F }
// or
struct Struct<'a> { f: Box<FnMut(uint) -> bool + 'a> }
```
- Using `|x, y| f(x, y)` for closure "borrows"
This comes up in recursive functions, consider the following (contrived) example:
``` rust
fn foo(x: uint, f: |uint| -> bool) -> bool {
//foo(x / 2, f) && f(x) // can't use this because `f` gets moved away in the `foo` call
foo(x / 2, |x| f(x)) && f(x) // instead "borrow" `f` in the `foo` call
}
```
If you attempt to do the same with unboxed closures you'll hit ""error: reached the recursion limit during monomorphization" (see #19596):
``` rust
fn foo<F>(x: uint, mut f: F) -> bool where F: FnMut(uint) -> bool {
foo(x / 2, |x| f(x)) && f(x)
//~^ error: reached the recursion limit during monomorphization
}
```
Instead you *should* be able to write this:
``` rust
fn foo<F>(x: uint, mut f: F) -> bool where F: FnMut(uint) -> bool {
foo(x / 2, &mut f) && f(x)
//~^ error: the trait `FnMut` is not implemented for the type `&mut F`
}
```
But as you see above `&mut F` doesn't implement the `FnMut` trait. `&mut F` *should* implement the `FnMut` and the above code *should* work, but due to a bug (see #18835) it doesn't (for now).
You can work around the issue by rewriting the function to take `&mut F` instead of `F`:
``` rust
fn foo<F>(x: uint, f: &mut F) -> bool where F: FnMut(uint) -> bool {
foo(x / 2, f) && (*f)(x)
}
```
This finally works! However writing `foo(0, &mut |x| x == 0)` is unergonomic. So you can use a private helper function to avoid this:
``` rust
// public API function
pub fn foo<F>(x: uint, mut f: F) -> bool where F: FnMut(uint) -> bool {
foo_(x, &mut f)
}
// private helper function
fn foo_<F>(x: uint, f: &mut F) -> bool where F: FnMut(uint) -> bool {
foo_(x / 2, f) && (*f)(x)
}
```
Closes#14798
---
There is more cleanup to do: like renaming functions/types from `unboxed_closure` to just `closure`, removing more dead code, simplify functions which now have unused arguments, update the documentation, etc. But that can be done in another PR.
r? @nikomatsakis @aturon (You probably want to focus on the deleted/modified tests.)
cc @eddyb
- the self type includes some local type; and,
- type parameters in the self type must be constrained by a local type.
A type parameter is called *constrained* if it appears in some type-parameter of a local type.
Here are some examples that are accepted. In all of these examples, I
assume that `Foo` is a trait defined in another crate. If `Foo` were
defined in the local crate, then all the examples would be legal.
- `impl Foo for LocalType`
- `impl<T> Foo<T> for LocalType` -- T does not appear in Self, so it is OK
- `impl<T> Foo<T> for LocalType<T>` -- T here is constrained by LocalType
- `impl<T> Foo<T> for (LocalType<T>, T)` -- T here is constrained by LocalType
Here are some illegal examples (again, these examples assume that
`Foo` is not local to the current crate):
- `impl Foo for int` -- the Self type is not local
- `impl<T> Foo for T` -- T appears in Self unconstrained by a local type
- `impl<T> Foo for (LocalType, T)` -- T appears in Self unconstrained by a local type
This is a [breaking-change]. For the time being, you can opt out of
the new rules by placing `#[old_orphan_check]` on the trait (and
enabling the feature gate where the trait is defined). Longer term,
you should restructure your traits to avoid the problem. Usually this
means changing the order of parameters so that the "central" type
parameter is in the `Self` position.
As an example of that refactoring, consider the `BorrowFrom` trait:
```rust
pub trait BorrowFrom<Sized? Owned> for Sized? {
fn borrow_from(owned: &Owned) -> &Self;
}
```
As defined, this trait is commonly implemented for custom pointer
types, such as `Arc`. Those impls follow the pattern:
```rust
impl<T> BorrowFrom<Arc<T>> for T {...}
```
Unfortunately, this impl is illegal because the self type `T` is not
local to the current crate. Therefore, we are going to change the order of the parameters,
so that `BorrowFrom` becomes `Borrow`:
```rust
pub trait Borrow<Sized? Borrowed> for Sized? {
fn borrow_from(owned: &Self) -> &Borrowed;
}
```
Now the `Arc` impl is written:
```rust
impl<T> Borrow<T> for Arc<T> { ... }
```
This impl is legal because the self type (`Arc<T>`) is local.
The earlier collections stabilization did not cover the modules
themselves. This commit marks as stable those modules whose types have
been stabilized.
